Radiation in the range 2900 to 3150 A exerts the most profound effects on the human body. The production of vitamin D in the skin by irradiation of the provitamin 7-dehydrocholesterol, the generation of a more or less intense and often painful erythema followed by reversible pigmentation, the wrinkling, creasing, and aging of the skin all result from different degrees of exposure of the human body to sunlight. Aside from these "normal" responses, there are also a considerable number of skin and systemic malfunctions which can be adversely affected by irradiation with ultraviolet and visible range electromagnetic radiation. (Kreps and Goldemberg, Cosmetics: Science and Technology Vol. 1, Balsam, et al Ed., John Wiley & Sons, Inc., 1972 p. 247).
Clinicians concerned with these damaging effects recommend the more widespread use of ultraviolet screening preparations as a preventive measure on a continuing basis, at least for fair-complexioned individuals, not only to prevent cancer but to prevent premature aging and wrinkling of the skin as well. (Kreps and Goldemberg, p. 256).
The only practical technique for preventing sunburn is to limit the exposure of the skin to the erythemogenic range of electromagnetic radiation to dosages less than those required to produce the harmful effect. This may be accomplished by avoiding exposure of the individual for more than the shortest period of time. Such limitations do not, however, fit into the designs of persons who enjoy outdoor activity and admire darkening of the skin acquired through exposure to sunlight. Avoidance of exposure is not necessary because the effects of the exposure may be reduced readily by the use of cosmetic agents which, applied to the skin, attenuate the dosage of erythemogenic radiation reaching the skin. This desired attenuation may be achieved either by reflection of the radiation by a coating on the skin or by absorption of the erythemogenic energy by the coating before it can fall on the surfaces of the skin. (Kreps and Goldemberg, p. 262)
The present invention is concerned with absorption of the erythemogenic energy by a coating before it reaches the skin. Aside from the proper electronic structure to establish the wavelength of maximum absorptivity in the erythemal range, the successful screening material must also have a distinct cutoff so that it transmits wavelengths above the erythemal range, it must show resistance to chemical and photochemical changes in structure, it should undergo minimum absorption through the skin, it should be sufficiently soluble in conventional cosmetic vehicles but it should be relatively insoluble in water or perspiration, it should be essentially free of toxic, irritating or sensitizing properties, and it should be essentially self-plasticizing in use. (Kreps and Goldemberg, p. 268).
Customary vehicles for sunscreens are hydroalcoholic lotions, water-in-oil or oil-in-water emulsions, and oily lotions. It is essential that the sun-screening compound shall be dissolved or dispersed easily and permanently in the vehicle. Once it is spread on the skin, it should remain in place as a continuous film, closely adhering to the surface, and it should resist washing off either by perspiration or by immersion in fresh or salt water.
Screens with limited solubility in the vehicle may crystallize during shelf storage in the container, or they may be thrown out of solution during shipment through cold-weather areas. Some materials may be unusuable because they cannot be dissolved in effective concentrations. The concentrations are often higher than the nominal concentration of the total suntan preparation; the concentration of the screen in the nonvolatile residual film left on the skin after evaporation of the volatile ingredients may be as high as 30 to 40% and higher in the case of simple hydroalcoholic lotions. If the screen crystallizes or separates from the other ingredients at these concentrations, the coverage of the skin may be patchy and the resulting protection afforded by the preparation may be incomplete.
The solubility of a screen in water or in perspiration generally implies that the film can be washed off the skin easily, with a consequent loss of protection. The film deposited on the skin is so small, however, that even slightly water-soluble screens are readily removed. After a few minutes of swimming on a fresh water pool, 90 to 99% of the screen is removed from the skin, and the amount of screen that remains is not effective. This is essentially independent of the nature of the carrier vehicle, although it appears that the swim removes an oily film more completely than it does a film deposited from a hydroalcoholic lotion. (Kreps and Goldemberg, p. 270).
Varied cosmetic bases have been employed to carry the sunscreen, and the makeup of the vehicle affects the efficacy of the product. Creams, cream lotions, oils, gels, hydroalcoholic lotions, jellies, lipsticks, and aerosol foams and sprays are common formulations which comprise the primarily cosmetic suntan preparations. Dermatologists often prefer ointments as the vehicle of choice, and they have also recommended the inclusion of sunscreens in makeup and hair preparations to prevent solar damage. (Kreps and Goldemberg, p. 285).
The formulations set forth below reveal gels and jellies that are prominent in the prior art.